The present invention relates to modulating PGE synthase activity. In particular, the present invention is based on identification of PGE synthase and DNA encoding it, providing for assays for substances able to modulate, especially inhibit, PGE synthase activity. PGE is a potent compound known to produce inflammation (symptoms including fever and pain), and inhibition of its production may be used in treatment of inflammation, arthritis, cancer, Alzheimer""s disease, in modulating apoptosis, and treating pain.
Prostaglandin endoperoxide H2 (PGH2) is formed from arachidonic acid by the action of cyclooxygenase (cox)-1 or -2. Cox-1 is constitutively expressed in many cells and tissues such as platelets, endothelium, stomach and kidney whereas the cox-2 protein can be induced by proinflammatory cytokines like interleukin-1xcex2 at sites of inflammation. For recent reviews on cox see Smith, W. (1997) Advances in Experimental Medicine and Biology 400B, 989-1011; Herschman, H. R. (1996) Biochimica et Biophysica Acta 1299, 125-40; Dubois, R., et al. (1998) Faseb J. 12, 1063-1073. Downstream of the cyclooxygenases, their product PGH2 can be further metabolized into the various physiologically important eicosanoids e.g. PGF2xcex1, PGE2, PGD2, PGI2 (prostacyclin) and thromboxane (TX) A2 (Smith, W. L. (1992) Am. J. Physiol. 263, F181-F191).
The mechanism for the biosynthesis of PGE2, and PGF1xcex1 (formed using dihomo-xcex3-linolenic acid instead of arachidonic acid) (Hamberg, M. and Samuelsson, B. (1967) J. Biol. Chem. 242, 5336-5343) by sheep vesicular glands was postulated to proceed via a cyclic endoperoxide (Samuelsson, B. (1965) J. Am. Chem. Soc. 87, 3011-3013) later designated PGH2 (Hamberg, M. and Samuelsson, B. (1973) Proc. Natl. Acad. Sci. USA 70, 899-903; Hamberg, M., et al. (1974) Proc. Natl. Acad. Sci. USA 71, 345-349; Nugteren, D. H. and Hazelhof, E. (1973) Biochim. Biophys. Acta 326, 448-461). In short, the reactions catalyzed by the cyclooxygenases involve a stereospecific abstraction of the 13-pro-S hydrogen atom from arachidonic acid. This leads to the formation of a carbon radical that is trapped by molecular oxygen at position C-11, formation of the 9,11-endoperoxide and the bond between the C-8 and C-12 positions with trans aliphatic side chains, radical rearrangement to C-15 and reaction with a second molecule of oxygen. In the next step the resulting peroxy group at C-15 is reduced to a hydroperoxy group and PGG2 is formed. This hydroperoxy group can subsequently be reduced by the peroxidase activity of cyclooxygenase (in the presence of a reducing agent e.g. glutathione) thus forming PGH2 
The enzyme/s responsible for the isomerization of PGH2 into PGE2 are not well known. Attempts have been made to purify the microsomal PGE synthase from ovine and bovine seminal vesicles, an organ known to contain high PGE synthase activity (Ogino, N., et al. (1977) Journal of Biological Chemistry 252, 890-5; Moonen, P., et al. (1982) Methods in Enzymology 86, 84-91). These studies have shown that the microsomal PGE synthase can be solubilized and partly purified. The enzyme activity was also dependent on glutathione but rapidly inactivated during the course of purification. Two monoclonal antibodies designated IGG1 (hei-7) and IGG1 (hei-26) raised against partly purified PGE synthase from sheep seminal vesicles, could immunoprecipitate two proteins from sheep seminal vesicles with molecular masses of 17.5 and 180 kDa, respectively (Tanaka, Y., et al. (1987) J. Biol. Chem. 262, 1374-1381). Both these precipitated proteins were found to possess glutathione dependent PGE synthase activity but no glutathione S-transferase activity. Interestingly, the IGG1 (hei-7) antibody also caused co-precipitation of cyclooxygenase, demonstrating that the 17.5 kDa protein and the cox proteins were on the same side of the microsomal membranes. The 17.5 kDa protein showed a Km for PGH2 of 40 xcexcM, similar to what has been described by others investigating the microsomal PGE synthase (Moonen, P., et al. (1982) Methods in Enzymology 86, 84-91). In contrast, the larger protein demonstrated a Km for PGH2 of 150 xcexcM. Additional proteins, belonging to the cytosolic glutathione S-transferase superfamily, have also been described to possess PGE, PGD and PGF synthase activities (Urade, Y., et al. (1995) J. Lipid Med. 12, 257-273). Recently, a microsomal 16.5 kDa protein was purified from sheep seminal vesicles possessing glutathione dependent PGF2xcex1, synthase activity (Burgess, J. R. and Reddy, C. C. (1997) Biochem. and Mol. Biol. Int. 41, 217-226). The enzyme (prostaglandin endoperoxide reductase) could also catalyze the reduction of cumene hydroperoxide whereas, 1-chloro-2,4-dinitrobenzene (typical substrate for various glutathione S-transferases) was not a substrate. Microsomal PGE synthase activity was also measured in various rat organs (Watanabe, K., et al. (1997) Biochemical and Biophysical Research Communications 235, 148-52) and high glutathione dependent activity was found in the deferens duct, genital accessory organs and kidney. Glutathione independent microsomal PGE synthase activity was observed in heart, spleen and uterus.
The enzyme responsible for PGE biosynthesis therefore provides a novel target for drug development in order to treat various inflammatory disorders. However, as is apparent from the preceding discussion, no-one has previously succeeded in providing pure PGE synthase nor the means to provide it.
Oxford Biomedical sells a partially purified preparation of ovine PGE synthase (Catalog Number PE 02). Analysis of that preparation indicates it is rather crude, including a complex mixture of numerous components.
Particular difficulties in purifying PGE synthase include the fact that the protein is a membrane protein, in general very hard to purify to homogeneity and the fact that its enzyme activity is very unstable after solubilization. Also, the work described herein demonsrates that the protein possesses very high enzyme activity, providing indication that the amounts of protein are very low within cells, adding to the difficulty of purification.
Urade et al. (1995) J. Lipid Med. 12, 257-273, notes xe2x80x9clittle is known about the properties of PGE synthasexe2x80x9d. Even more recently, William Smith in xe2x80x9cMolecular Biology of Prostanoid Biosynthetic Enzymes and Receptorsxe2x80x9d, Advances in Experimental Medicine and Biology, 400B: 989-1011, published in 1997, noted xe2x80x9cThe PGE synthase story has been a perplexing onexe2x80x9d, pointing out that PGE formation has not been attributed to a unique protein.
The work of the present inventors described below demonstrates that human PGE synthase is a member of a protein superfamily consisting of membrane associated 14-18 kDa proteins involved in eicosanoid and glutathione metabolism. PGE synthase demonstrates 38% identity on the amino acid sequence level with microsomal glutathione S-transferase 1. The human cDNA sequence as well as the predicted amino acid sequence were deposited in 1997 in public databases under the name of MGST1-L1 (GenBank accession number AF027740) as well as a p53 induced PIG12 (GenBank accession number AF010316). No function has previously been ascribed to these cDNA sequences.
Polyak et al. (1997) Nature 389: 300-305 identified what they called xe2x80x9cPIG12xe2x80x9d by cloning sequences of which expression was upregulated by P53. They state xe2x80x9cPIG12 is a novel member of the microsomal glutathione S-transferase family of genesxe2x80x9d, but identify not actual function. There is certainly no suggestion that their PIG12 was actually human PGE synthase.
In summary, no-one has previously provided PGE synthase in any form or quantity that would allow for amino acid sequencing to provide a potential starting point for attempted cloning of a coding sequence. Furthermore there was no suggestion that the sequence on the databases which the present inventors have now demonstrated to encode PGE synthase did actually encode PGE synthase.
In the light of the inventors"" work, the present invention provides in various aspects for use of purified PGE synthase in various contexts, in particular in assays and screening methods for substances able to modulate, especially inhibit, PGE synthase activity. The purified PGE synthase may be made by recombinant expression from encoding nucleic acid. It may be expressed in eukaryotic or prokaryotic expression systems and may lack native glycosylation. Substances identified as modulators of PGE synthase may be employed in control or treatment of inflammation, arthritis, cancer or other cellular growth abnormality, Alzheimer""s disease, in modulating apoptosis, and treating pain.